net: lantiq_xrx200: increase buffer reservation

If the user sets a lower mtu on the CPU port than on the switch,
then DMA inserts a few more bytes into the buffer than expected.
In the worst case, it may exceed the size of the buffer. The
experiments showed that the buffer should be a multiple of the
burst length value. This patch rounds the length of the rx buffer
upwards and fixes this bug. The reservation of FCS space in the
buffer has been removed as PMAC strips the FCS.

Fixes: 998ac358 ("net: lantiq: add support for jumbo frames")
Reported-by: Thomas Nixon <tom@tomn.co.uk>
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/lantiq_xrx200.c

@@ -71,6 +71,8 @@ struct xrx200_priv {
 	struct xrx200_chan chan_tx;
 	struct xrx200_chan chan_rx;
 
+	u16 rx_buf_size;
+
 	struct net_device *net_dev;
 	struct device *dev;
 
@@ -97,6 +99,16 @@ static void xrx200_pmac_mask(struct xrx200_priv *priv, u32 clear, u32 set,
 	xrx200_pmac_w32(priv, val, offset);
 }
 
+static int xrx200_max_frame_len(int mtu)
+{
+	return VLAN_ETH_HLEN + mtu;
+}
+
+static int xrx200_buffer_size(int mtu)
+{
+	return round_up(xrx200_max_frame_len(mtu), 4 * XRX200_DMA_BURST_LEN);
+}
+
 /* drop all the packets from the DMA ring */
 static void xrx200_flush_dma(struct xrx200_chan *ch)
 {
@@ -109,8 +121,7 @@ static void xrx200_flush_dma(struct xrx200_chan *ch)
 			break;
 
 		desc->ctl = LTQ_DMA_OWN | LTQ_DMA_RX_OFFSET(NET_IP_ALIGN) |
-			    (ch->priv->net_dev->mtu + VLAN_ETH_HLEN +
-			     ETH_FCS_LEN);
+			    ch->priv->rx_buf_size;
 		ch->dma.desc++;
 		ch->dma.desc %= LTQ_DESC_NUM;
 	}
@@ -158,21 +169,21 @@ static int xrx200_close(struct net_device *net_dev)
 
 static int xrx200_alloc_skb(struct xrx200_chan *ch)
 {
-	int len = ch->priv->net_dev->mtu + VLAN_ETH_HLEN + ETH_FCS_LEN;
 	struct sk_buff *skb = ch->skb[ch->dma.desc];
+	struct xrx200_priv *priv = ch->priv;
 	dma_addr_t mapping;
 	int ret = 0;
 
-	ch->skb[ch->dma.desc] = netdev_alloc_skb_ip_align(ch->priv->net_dev,
-							  len);
+	ch->skb[ch->dma.desc] = netdev_alloc_skb_ip_align(priv->net_dev,
+							  priv->rx_buf_size);
 	if (!ch->skb[ch->dma.desc]) {
 		ret = -ENOMEM;
 		goto skip;
 	}
 
-	mapping = dma_map_single(ch->priv->dev, ch->skb[ch->dma.desc]->data,
-				 len, DMA_FROM_DEVICE);
-	if (unlikely(dma_mapping_error(ch->priv->dev, mapping))) {
+	mapping = dma_map_single(priv->dev, ch->skb[ch->dma.desc]->data,
+				 priv->rx_buf_size, DMA_FROM_DEVICE);
+	if (unlikely(dma_mapping_error(priv->dev, mapping))) {
 		dev_kfree_skb_any(ch->skb[ch->dma.desc]);
 		ch->skb[ch->dma.desc] = skb;
 		ret = -ENOMEM;
@@ -184,7 +195,7 @@ static int xrx200_alloc_skb(struct xrx200_chan *ch)
 	wmb();
 skip:
 	ch->dma.desc_base[ch->dma.desc].ctl =
-		LTQ_DMA_OWN | LTQ_DMA_RX_OFFSET(NET_IP_ALIGN) | len;
+		LTQ_DMA_OWN | LTQ_DMA_RX_OFFSET(NET_IP_ALIGN) | priv->rx_buf_size;
 
 	return ret;
 }
@@ -356,6 +367,7 @@ xrx200_change_mtu(struct net_device *net_dev, int new_mtu)
 	int ret = 0;
 
 	net_dev->mtu = new_mtu;
+	priv->rx_buf_size = xrx200_buffer_size(new_mtu);
 
 	if (new_mtu <= old_mtu)
 		return ret;
@@ -375,6 +387,7 @@ xrx200_change_mtu(struct net_device *net_dev, int new_mtu)
 		ret = xrx200_alloc_skb(ch_rx);
 		if (ret) {
 			net_dev->mtu = old_mtu;
+			priv->rx_buf_size = xrx200_buffer_size(old_mtu);
 			break;
 		}
 		dev_kfree_skb_any(skb);
@@ -505,7 +518,8 @@ static int xrx200_probe(struct platform_device *pdev)
 	net_dev->netdev_ops = &xrx200_netdev_ops;
 	SET_NETDEV_DEV(net_dev, dev);
 	net_dev->min_mtu = ETH_ZLEN;
-	net_dev->max_mtu = XRX200_DMA_DATA_LEN - VLAN_ETH_HLEN - ETH_FCS_LEN;
+	net_dev->max_mtu = XRX200_DMA_DATA_LEN - xrx200_max_frame_len(0);
+	priv->rx_buf_size = xrx200_buffer_size(ETH_DATA_LEN);
 
 	/* load the memory ranges */
 	priv->pmac_reg = devm_platform_get_and_ioremap_resource(pdev, 0, NULL);